100% Wildflower seed mixes should be sown and treated in a similar way to grass and wildflower mixes. It is generally difficult to get wildlfower seed to establish into existing grassland. Normally our advice is to clear the ground and start again.
If the grass is very poor then it may possibly work. You will need at least 50% bare soil. Cut the grass back very short then scarify or harrow the ground hard. clear as much dead thatch away as possible. Broadcast the seed at 1.0 to 1.5 grams per square metre. Sow in Spring, March through April. Or in Autumn, August through to early October.
Perennial wildflower species are slow to establish and many you would not see until at least the second year. Each year treat as a meadow. in early Autumn, (mid August) cut back the area. In the first year to a height of about 4 to 5 inches. In following years you can cut it back to about 1 inch in height. Perennials should then flower again each year.